1. Field Of The Invention
This invention relates to particulate thermosetting adhesives and methods for their preparation. More specifically, the invention relates to methods for preparing solid particles comprising an inner carrier substance and an outer coating of thermosetting resin.
2. Description Of The Prior Art
Particulate thermosetting adhesives in the form of finely divided powders are extensively used as binders in the manufacture of many industrial composites. Powdered phenolic adhesives are used in the manufacture of composite wood products such as waferboard, particleboard, fiberboard and the like. Powdered melamine adhesives are used in the manufacture of molded wood products and molding compounds. A description of the processes involved in the manufacture of such composites can be found in "Phenolic Resins" by Andre Knop and Louis A. Pilato, Springer Verlang Berlin Heidelberg, 1985.
A common factor in all these processes is the application of sufficient heat to the composite to allow the thermosetting adhesive powder contained therein to melt, thereby wetting the substrate surface and subsequently solidifying to an infusible mass bonding the wetted surfaces together.
Thus, thermosetting adhesives suitable for the manufacture of industrial composites must exhibit heat softening properties to effectively and uniformly coat and wet the substrate surface. However, in the manufacture of composites containing porous substrates such as wood or cotton based materials, adhesives with heat softening properties will not only coat and wet the substrate surfaces, but will also penetrate and absorb through these surfaces. This effectively reduces the amount of adhesive available at the substrate interface, thereby weakening the strength of the composite. While an increase in the amount of adhesive used in the composite would compensate for this loss, such measures are wasteful and result in increased costs.
U.S. Pat. No. 4,424,300 to Udvardy et al discloses a method of making a spray dried phenolic resole resin having heat softening and fast cure properties. The heat softening properties are imparted by incorporating in a separate step a novolac resin into a resole resin prior to spray drying. While this method provides for improved heat softening properties of the adhesive, it also results in increased penetration of the adhesive into porous substrates and consequently reduced strength of the composite. Additionally, the manufacture in separate steps of a novolac and a resole, with subsequent blending of the two is time consuming and wasteful.
U.S. Pat. No. 4,098,770 to Berchem et al discloses a method of making a spray dried phenolic resin with a non-phenolic polyhydroxy compound added thereto prior to spray drying. The non-phenolic polyhydroxy compound significantly improves the heat softening properties of the resin, but also results in increased penetration of the resin in the case of porous substrates. Additionally, because of the humectant nature of such non-phenolic polyhydroxy compounds, spray drying of resins containing such compounds is difficult and wasteful.
U.S. Pat. No. 3,931,070 to Bond et al discloses a phenol-formaldehyde resin plywood adhesive containing at least 1% of a sodium lignosulfate and up to 1% of a trialkyl phosphate. The addition of trialkyl phosphate and lignosulfate to the phenol-formaldehyde adhesive containing extenders and fillers, especially amylaceous and proteinaceous materials modifies the surface properties of the phenolic adhesive mixture to enhance the curtainability or film forming properties to provide an even, uniform coating of the adhesive on a veneer.
A significant amount of prior art also relates to the use of lignins or waste sulfite liquor as a binder additive for cellulosic material, such as, U.S. Pat. No. 4,357,454 to Holmberg et al, U.S. Pat. No. 4,324,747 to Sudan et al, U.S. Pat. No. 4,303,562 to Hollis, Jr. et al, U.S. Pat. No. 4,127,544 to Allan, U.S. Pat. No. 4,105,606 to Forss et al, U.S. Pat. No. 3,886,101 to Felicetta et al, and U.S. Pat. No. 3,864,291 to Enkvist.